Iran's output of enriched uranium is stagnating, diplomats said Tuesday, suggesting that Teheran may be running short the material needed for producing nuclear fuel or the fissile core of warheads. The diplomats - who demanded anonymity because their information was confidential - emphasized that the possibility that Iran was running out of uranium oxide was only one of several possible explanations of why it had not increased its production of enriched uranium since May. But they said it seemed unlikely that the Islamic republic had deliberately decided to curb its production. They noted that despite the stagnation in output, Iran continued over the past three months to expand its capabilities by installing hundreds more of the centrifuges that spin uranium oxide into enriched uranium. Iran is under three sets of UN Security Council sanctions for refusing to freeze its uranium enrichment activities. These were imposed out of fears that Iran is using the pretext of building a peaceful nuclear program - including enrichment to low levels suited for making nuclear fuel - to eventually make weapons-grade enriched uranium. In its last report on Iran in June, the International Atomic Energy Agency said that nearly 5,000 centrifuges were operating at Iran's cavernous underground enriching facility at Natanz by May. Diplomats on Tuesday said that had expanded to about 6,000 of the machines by last month. The Washington-based Institute for Science and International Security recently estimated that even taking the 5,000-centrifuge figure as a basis, Iran could accumulate enough material to produce weapons-grade uranium for two warheads by February 2010. Iran steadfastly refuses to stop enriching despite the imposition of three rounds of economic, trade and financial sanctions by the UN Security Council. But it is believed to be dependent on imports of uranium oxide the feed the centrifuges, with domestic mining yielding only relatively small quantities of inferior ore.